Mix together the olives, the mayo, the onion, and the vinegar. Carefully pour juice from the olive jar into the mayo mixture, enough to thin it into a dressing that will coat the macaroni – without being too thick (not too precise, but that’s how Flora did it). If the dressing is too thin, add a little more mayo. Add salt and pepper to taste. Pour the dressing over the macaroni and mix carefully. Refrigerate for several hours before serving. After several hours, the macaroni will have absorbed a lot of the dressing. You can thin a bit more mayo with olive juice, and mix it in the salad to moisten it up before serving.